It isn't just about breaking a cycle, though not shaming addiction is helpful. It isn't just addicts that do drugs: I've met folks that do cocaine twice a year, for example. Few folks are addicted to LSD, and most folks only take MDMA occasionally. There is more than one way to take heroin, and folks do use occasionally. All drugs I wouldn't take are safer if folks know they are getting, strength and purity are verified as much as possible.
Legalisation takes some strength away from cartels, which hopefully means less human trafficking.